Synthesis, characterization, and calculated electronic structure of the crystalline metal-organic polymers [Hg(SC ₆H ₄S)(en)] _n and [Pb(SC ₆H ₄S)(dien)] _n

The reaction of Hg(OAc) ₂ with 1,4-benzenedithiol in ethylenediamine at 80 °C yields [Hg(SC ₆H ₄S)(en)] _n, while the reaction of Pb(OAc) ₂ with 1,4-benzenedithiol in diethylenetriamine at 130 °C yields [Pb(SC ₆H ₄S)(dien)] _n. Both products are crystalline materials, and structure determination by synchrotron X-ray powder diffraction revealed that both are essentially one-dimensional metal-organic polymers with -M-SC ₆H ₄S- repeat units. Diffuse reflectance UV-visible spectroscopy indicates band gaps of 2.89 eV for [Hg(SC ₆H ₄S)(en)] _n and 2.54 eV for [Pb(SC ₆H ₄S)(dien)] _n, while density functional theory (DFT) band structure calculations yielded band gaps of 2.24 and 2.10 eV, respectively. The two compounds are both infinite polymers of metal atoms linked by 1,4-benzenedithiolate, the prototypical molecule for single-molecule conductivity studies, yet neither compound has significant electrical conductivity as a pressed pellet. In the case of [Pb(SC ₆H ₄S)(dien)] _n calculations indicate fairly flat bands and therefore low carrier mobilities, while the conduction band of [Hg(SC ₆H ₄S)(en)] _n does have moderate dispersion and a calculated electron effective mass of 0.29·m _e. Hybridization of the empty Hg 6s orbital with SC ₆H ₄S orbitals in the conduction band leads to the band dispersion, and suggests that similar hybrid materials with smaller band gaps will be good semiconductors.